The present invention relates to apparatus for measuring torque on a rotating shaft and provides instrumentation and a method for carrying out such measurements without physically contacting the shaft and without attaching toothed gears, wheels, and other such apparatus or indicia to the shaft.
For many kinds of rotary machines, such as, for example, the large turbine-generators used for commercial power generation, it is desirable, and indeed frequently necessary, to monitor the torque developed between separate locations on the central rotating shaft. The torque generated on the rotating shaft is often oscillatory in nature, raising and falling as a result of the complex interaction of the various masses being driven by the shaft, and, in the case of a turbine-generator, also as a result of the interaction of the electrical power output with the connected electrical load. Torque between axially separated points on a shaft produces a degree of twist in the shaft whose magnitude depends on the torque and on the metallurgy and geometry of the shaft.
Continuous torque monitoring not only provides a permanent record for analysis to correlate shaft torque with various operating conditions of the machine, but it also provides an immediate indication that corrective action is required should the torque become excessive. Excessive torque is to be guarded against, of course, to prevent permanent damage to the shaft and to other portions of the machine in those extreme situations wherein excessive torque may cause the shaft to be broken.
A number of instrument systems have been developed over the years for continuously monitoring torque on a rotating shaft. Generally, these systems have required that some device be added to the shaft in order to detect shaft rotational speed as a fundamental measurement. Toothed gears for example are commonly placed on the shaft so that the passage of gear teeth past a fixed point provides an indication of shaft angular velocity. Light reflective strips applied to the rotating shaft are also known to be used to detect shaft speed. From the basic speed indication, in combination with other measurements or assumptions, a measure of torque is then obtainable as a derived quantity.
A direct measurement of torque can be gained by using two separate sets of such rotating indicia. For example, in U.S. Pat. No. 4,186,597 a digital torque meter is disclosed in which a pair of spaced-apart gears on the shaft provide signals whose phasing is compared as an indication of twist and torque on the shaft. In U.S. Pat. No. 3,934,459 to Wolfinger et al, shaft angular velocity from a shaft mounted toothed wheel provides the fundamental measurement and torque in various shaft sections is then derived by application of a mathematical modeling technique.
Although these prior art systems and techniques have generally performed their intended functions satisfactorily, their utility has been limited by the necessity of making some attachment to the rotating shaft. In many cases it is simply not practical to add a toothed wheel to a rotating shaft. Further, light reflecting strips and other such indicia become obscure with time and often are hidden by oil mists, deposited films, and so forth. The present invention is therefore directed toward apparatus and a method for measuring torque in a rotating shaft, which method and apparatus alleviates the aforementioned and other prior art problems.